Cutting tools, in particular tools for chip-forming metal machining, consist of a main body which is made for example of hard metal, cermet, ceramic, steel or high-speed steel. To increase the tool life or to improve the cutting properties, a single-layer or multi-layer wear protection coating made of hard materials is frequently applied to the main body by means of CVD or PVD processes. The PVD processes come in a number of different variants, such as magnetron sputtering, arc vapor deposition (arc PVD), ion plating, electron beam vapor deposition and laser ablation. Magnetron sputtering and arc vapor deposition are the PVD processes which are used most often for coating of tools. Individual PVD process variants, in turn, include a variety of modifications such as for example unpulsed or pulsed magnetron sputtering or unpulsed or pulsed arc vapor deposition, etc.
In arc vapor deposition (arc PVD) the arc generates very high temperatures in the order of several thousand degrees celsius at the target resulting in the desired evaporation or sublimation, respectively, of the target material for a deposition on the substrate. Around the point of impact of the arc areas of lower temperatures are generated, such as for example in the region from 500 to 1,000° C., where in some target metals, particularly low melting metals, such as aluminum, there occurs detachment of macro-particles, so called droplets, which are also co-deposited on the substrate. Such droplets lead to undesired roughness of the deposited coating compared to coatings produced for example by magnetron sputtering. In addition, the droplets result in a weakening of the coating, because in general they predominantly consist of pure metal and thereby exhibit a lower hardness and increased tendency for oxidation. It is therefore desired to reduce the droplet formation in the PVD arc vapor deposition process.
For specific metal machining operations, such as for example milling, turning and drilling, there exist particularly high demands on the tool. Important parameters for such tools are high hardness, high modulus of elasticity (E-modulus, Young's modulus) and a low surface roughness. Known cutting tools for the described applications comprise TiAlN coatings deposited in a PVD process, which typically have a modulus of elasticity below 400 GPa and a Vicker's hardness up to 3,500 HV. If such TiAlN coatings are deposited by the arc vapor deposition process, they have a tendency for the formation of droplets on and in the coating due to the low melting temperature of the aluminum, which has a disadvantageous effect on the performance of the coating. By appropriate selection of the parameters of the deposition process the hardness and the modulus of elasticity of PVD coatings can be increased, however, in general, this leads to high residual compressive stresses in the coating in the order significantly above 3 GPa, which have disadvantageous effects on the stability of the cutting edge. Under high load such a cutting edge tends to early chipping, and thereby, to a rapid wear of the tool.
There are commercially available tools having a PVD coating comprising a first multi-layer coat of TiN and TiAlN and a second multi-layer coat of TiSeN and AlCrN. The coating is deposited by two subsequent deposition processes to achieve a high coating thickness. After the deposition of the first multi-layer coat in a first deposition process the coated substrate is cooled down and subjected to a mechanical treatment wherein the residual compressive stresses of the first coat are changed, before the second multi-layer coat is then deposited in a further deposition process. The coating exhibits a high hardness, but, at the same time, also very high residual compressive stresses significantly above 3 GPa, which has a disadvantageous effect on the stability of the cutting edge, the uniform covering of the cutting edge and the performance of the tools.
WO 2006/041367 A1 describes a coated cutting tool consisting of a hard metal substrate and a coating which is deposited in the PVD process and comprises at least one coat of TiAlN having a thickness of 1.5 to 5 μm and a residual compressive stress >4 to 6 GPa. The TiAlN coat is said to adhere more effectively to the substrate compared with known coats. The very high residual compressive stresses have, however, a disadvantageous effect on the stability of the cutting edge.
EP 2 298 954 Al describes a method for producing a coated cutting tool wherein a hard material coating, for example TiAlN or TiAlCrN, is applied to a substrate by the PVD process, and wherein the bias voltage of the substrate being varied during the deposition process. The method is said to provide an improved wear resistance and a longer service life of the tool.